The present invention relates to the removal of mercury (Hg) from sulfuric acid (H.sub.2 SO.sub.4) solutions. More specifically, the invention provides a method that is not temperature or concentration dependent for removing both metallic and ionic mercury from commercially produced sulfuric acid.
The commercial production of sulfuric acid from sulfur dioxide (SO.sub.2) bearing gases inherently results in trace amounts of mercury, i.e. from about 2 to about 20 parts per million, being present in the final acid solution. Typically, the SO.sub.2 bearing gas is supplied to the acid plant from a roaster or smelter which burns off the sulfidic content of metallic ores and concentrates. Metallic ores and concentrates, for example those of lead, zinc and copper, naturally contain trace amounts of mercury in the form of HgS which decomposes during the roasting operation, liberating mercury which is vaporized and carried over to the acid plant with the SO.sub.2 bearing feed gas. Typically, 40% to 50% of the mercury carried over to the acid plant in the SO.sub.2 bearing gas from the roaster is found in the acid product.
For many years the presence of trace amounts of Hg in sulfuric acid has not been considered a problem. However, future governmental regulations may specify an allowable level of Hg in sulfuric acid that is below the amount now common in untreated acid. Typically, the Hg content of H.sub.2 SO.sub.4 produced from metallurgical SO.sub.2 bearing gas is from about 2 parts per million (ppm) to about 20 ppm. It is desirable to reduce the Hg content of such H.sub.2 SO.sub.4 solutions to less than 1 ppm, hereinafter sometimes referred to as a low Hg content or level.
Accordingly, the present invention provides a method for the substantial reduction of typical Hg levels in H.sub.2 SO.sub.4 produced from SO.sub.2 bearing gases from metallurgical processes, such as roasting and sintering sulfidic ores, to low levels of Hg. The new method is economical to use, does not require a large capital investment and is not temperature or concentration dependent.
It is known in the art to produce H.sub.2 SO.sub.4 having a low Hg content by either removing the Hg from the SO.sub.2 feed gas prior to the acid plant or from the product acid or from both. A printed publication entitled "Mercury Control For Sulfuric Acid Manufacture" from the book "World Mining and Metals Technology" describes commercially used methods for removing Hg from the SO.sub.2 bearing gas and from the product acid, either independently or in combination, depending upon the starting material and the desired results.
The removal of Hg from metallurgical gas, as described in the aforementioned publication, provides for the injection of modest amounts of H.sub.2 S at selected point in the gas purification system. The Hg vapor is converted to HgS which is removed from the gas stream by coke filters. Periodically, the mercury-laden coke is removed from the filters and treated for mercury recovery. This process is also described in U.S. Pat. No. 3,855,387.
The aforementioned publication also describes, and U.S. Pat. No. 3,875,287 discloses a process for reducing the Hg content of sulfuric acid to desired low levels, i.e. about 0.5 ppm, by adding potassium and other iodides in powder form to the acid to form nascent iodine that reacts with Hg to produce mercury iodide. The mercury iodide is insoluble in the acid and precipitates out. This process is temperature and concentration sensitive in that the formation of HgI.sub.2 does not take place at commercially feasible rates at temperatures higher than about 30.degree. C. or with acid concentrations lower than about 70% by weight.
The removal of mercury contained in sulfuric acid as a sulfide precipitate is taught in German Pat. No. 1,054,972. As disclosed in German Pat. No. 1,216,263, elemental sulfur is also known as a precipitating agent to remove mercury from sulfuric acid.